1. Field of the Invention
The present invention relates to a spark plug electrode, which is produced from an electrode material on a nickel basis.
2. Description of Related Art
Because of the continuous refinement of motor vehicle engines and their components in an effort to increase the performance and engine power, ever greater demands are placed on the materials of the engine components as well. Especially the components that play an important role in the ignition of the fuel mixture, the spark plugs, and especially the spark plug electrodes, are exposed to high stressing. In the ignition process, an ignition system controlled by the engine periodically generates a high voltage, which is discharged in a spark arc-over between the two electrodes of the spark plugs. The produced spark then ignites the compressed air-fuel mixture. In the process, the spark plug is subjected to permanent stressing by extremely high temperatures. To prevent the engine performance from decreasing in long-term operation as a result of leaking, poorly igniting or overheating spark plugs, the materials for producing spark plug electrodes for internal combustion engines are subject to ongoing further development.
In general, nickel alloys are used as base material for spark plug electrodes, because nickel not only has high melting temperatures, which are an absolute requirement for the temperature stability of the alloy, but also high resistance to corrosion. Although materials produced from pure noble metals or on the basis of noble metals, such as platinum or platinum alloys including iridium, exhibit increased wear resistance to spark-erosive attacks and thus provide a very high service life of the electrodes, spark plug electrode materials of platinum do not constitute a viable alternative to conventional nickel alloys for economic reasons in view of the enormous cost.
The resistance of nickel alloys manifests itself in low erosion losses, i.e., material removal from the electrode, induced by the reciprocal effect of the electric arc with the electrode surface, and in high oxidation and corrosion resistance. The corrosion resistance can be increased even further by metal additives such as aluminum, manganese, chromium and the like. Moreover, adding silicon to the nickel-base alloy increases the high-temperature oxidation resistance.
From the published German patent document DE 39 16 378 A1, an alloy on nickel basis is known for use in spark plug electrodes for internal combustion engines, which is essentially made up of nickel, silicon, manganese and aluminum, the silicon weight component amounting to 0.1 to 1.5 weight %, the manganese component to 0.1 to 0.65 weight %, and the aluminum component to 3.1 to 5 weight %. Chromium up to 2 weight %, or Y or an element of the rare earths up to 0.5 weight % may be contained as additional components. According to the explanations in this printed publication, nickel alloys are obtained that exhibit good oxidation and corrosion resistance at increased temperatures as well as increased resistance to spark erosion as a result of their stability.
The higher stability does indeed increase the oxidation and corrosion resistance, but it also promotes chipping of material at the surface of the electrode, which is caused by the extreme thermal stressing in the spark arc-over between the center and the ground electrode as a result of the reduced elasticity. Furthermore, such a compact material is complex, expensive to produce and difficult to process.
It should be stated at the outset that, unless expressly denoted otherwise, all of the following atom % indications always relate to the total composition of the electrode material.